1. Field of the Invention
The present invention relates to an optical amplifier module, which amplifies light to be amplified, and a laser light source apparatus comprising an optical amplifier module of this kind.
2. Related Background Art
Laser light sources which output light having a small spot size are required in industries such as ultrafine processing. In order to achieve practical use of a laser light source of this kind in an industrial application, and in particular, the processing of semiconductors and electrical components, it is necessary to shorten the pulse and raise the output of the laser light. In other words, it is necessary to achieve a laser light source having a pulse width of the nanosecond order which outputs light having a peak power exceeding a kW.
In order to achieve a laser light source which outputs light having a small spot size, a laser light source of a type in which the actual optical amplification medium forms a waveguide structure can be adopted. However, in the case of a laser light source of this kind, it is necessary to adopt a waveguide structure having a length of at least several meters, and therefore propagation delay occurs. As a result of this, in a laser light source having a resonance structure, such as a Q switch, the pulse width reaches 100 ns.
In order to resolve the aforementioned problem and shorten the pulse of the laser light to a pulse width of 10 ns or 1 ns, or lower, a Master Oscillator Power Amplifier (MOPA) structure is used in which a mode-locked solid-state laser light source, or a directly modulated laser diode is employed as a seed light source and the output power from this source is amplified in a single pass. A MOPA structure which amplifies light by using a directly modulated laser diode as a seed light source is employed in particular with a view to reducing costs and raising the freedom of the pulse cycle.
On the other hand, in order to raise the laser light output, since there exists no level above level 2 relating to lasing transitions, various methods are employed which involve doping the optical waveguide region with ytterbium (Yb) that does not produce absorption through up-conversion, as well as suppressing photodarkening.
The technology described above is explained in Japanese Patent Application Laid-open No. 2007-194501 (Document 1) and U.S. Patent Application Publication No. 2007-0053400 (Document 2), for example.
Document 1 discloses an optical amplifier module which uses a Yb-doped fiber (hereinafter referred to as YbDF) having a low Yb element concentration in the portion of high population inversion ratio, and uses a YbDF having a low Yb element concentration in the portion of low population inversion ratio. More specifically, Document 1 discloses an optical amplifier module comprising a first optical amplification medium having a core region doped with Yb, and a second optical amplification medium fusion spliced to the first optical amplification medium and having a core region doped with Yb at a higher concentration than the first optical amplification medium, wherein light to be amplified which is inputted to the first optical amplification medium is amplified respectively by the first and second optical amplification media.
Furthermore, Document 2 discloses a fiber laser light source comprising a first fiber section doped with Yb at a relatively low concentration and a second fiber section doped with Yb at a relatively high concentration, wherein light to be amplified which is inputted to the first fiber section is amplified respectively by the first and second fiber sections.